A non-limiting example of a technical field in which the present invention can be applied is the aircraft industry. Generally, aircrafts are painted. This cover with polymer layers is used for decorative, anti-erosion, anti-corrosion, air resistance reduction and other purposes. In order not to increase the weight of the primary structure unnecessarily, the applied coating must be even and not too thick, so as not to impair the hardening process and to avoid the formation of wrinkles, bubbles and/or droplets, and the occurrence of other coating defects. It must not be too thin, if the desired functional and/or visual effects are to be achieved, for example having a minimal opaqueness. Therefore, the thicknesses have to be monitored within close tolerances using measurement equipment, during production and also during maintenance procedures. Typical common thicknesses are in the range of 0.05 mm to 0.5 mm.
Known from the prior art are purely mechanically acting coating thickness measurement schemes and units, with which the determination of the material thickness of the applied polymer coating takes place, for example, by means of the measurement of the penetration path of a measurement body. Such coating thickness measurement units do on the one hand not allow a non-destructive measurement and on the other hand the measurement accuracy that can be achieved is inherently limited in principle, in particular in the case of low material thicknesses of polymer coatings.
Furthermore coating thickness measurement units are known in which the measurement of the material thickness of a polymer coating takes place through eddy current measurements with the introduction of electric coils. While measurement units of this type do allow a non-destructive measurement with a sufficiently high accuracy—after calibration has taken place—they have the disadvantage that the substrate with the polymer coating must feature a substantially high electrical conductivity. Accordingly these units are only suitable for the measurement of the material thickness of a polymer coating on a substrate formed from a composite material to a very limited extent. Such composite material as a rule has only a medium electrical conductivity, as for example in the case of a carbon fibre reinforced epoxy resin. In addition to the medium conductivity this material is anisotropic due to the distinct direction of fibres and rovings.
The limitation of the thickness measurement based on eddy currents to good conductors is caused by the skin depth which should be considerably smaller than the dielectric layer thickness. This does not hold for the medium electrically conducting carbon fibre reinforced plastic, even at the highest normally used eddy current frequencies of about 5 MHz. The skin depth in the substrate becomes too high. It can be reduced when operating at considerably higher frequencies, namely at microwave frequencies. Therefore there were several attempts for microwave based thickness measurements in the past.
From the German utility model DE 1 792 402 a device for the non-destructive measurement of a coating thickness of a medium on a base material is known, in which the measurement is based on the evaluation of the phase displacement that ensues with the passage of microwaves through the medium and their subsequent reflection at the base material. However, this coating thickness measurement unit of prior art only allows a measurement of the thickness of a coating located on a purely metallic base material. The proposal according to DE 1 792 402 is a rather bulky setup and the raw data depend on the dielectric constant of the measured coating. This system cannot be used for the intended applications.
The above mentioned phase displacement per unit coating thickness can be increased by using a coaxial reflection probe together with special circuit improvements, as known from document US 2011/0062965 A1. The coaxial probe described in US 2011/0062965 A1 generates a circular symmetric field and is therefore well suited to measure anisotropic substrates, i.e. no turning of the probe into a certain angle with respect to the carbon fibre direction is necessary. However, the probe produces an electric microwave field within the paint and therefore the thickness data are depending on the dielectric constant of the dielectric layer. Furthermore the momentarily irradiated area of dielectric layer in the substrate is rather small so that the result can vary over the surface when the substrate is too inhomogeneous. This can especially be the case on substrates with metallic mesh for lightning protection.
In patents U.S. Pat. No. 6,184,694 B1 and U.S. Pat. No. 6,297,648 B1 the use of reference cavities and frequency counters are proposed. The U.S. Pat. No. 7,173,435 B1 generally describes a device that is proposed to use two antennas with separate transmitter and receiver. The disclosures of U.S. Pat. No. 6,184,694 B1, U.S. Pat. No. 6,297,648 B1 and U.S. Pat. No. 7,173,435 do not describe the essential part in detail, i.e. the cavity, but only the surrounding circuitry. They use two antennas with reference cavities, frequency counters and/or separate transmitters and receivers. This makes the systems rather bulky.
In the U.S. Pat. No. 7,898,265 B2 a cavity resonator is used measuring in the transmission method with separate transmitter and receiver and with separate antennas. The use of the TM011 mode in a circular cylindrical cavity resonator is proposed. However, also here a transmission method is used with two antennas and separate transmitter and receiver which make the system rather bulky. The proposed TM011 mode has a circular cylindrical electromagnetic field and therefore it is not necessary to adjust a probe according to fibre direction.
However, the TM011 mode has significant microwave wall currents in axial direction which also flow in the undisturbed cavity from the tube segment to the end plates. As one of the end plates is constituted by the sample consisting of a substrate of medium conductivity with or without an isolating dielectric layer these currents are heavily disturbed. This causes significant contact problems when placing the microwave cavity on the device to be tested. Extremely small changes in the alignment will produce significant changes in the displayed data. The proposed groove choke will solve this problem only partially and will increase the size of the probe.
Furthermore the TM011 mode has a non-zero electric field which is perpendicular to the substrate plane and thus penetrates the dielectric layer to be measured. Therefore the data which are generated by the system depend on the dielectric constant of the paint. Furthermore it should be noted that without special precautions the useable bandwidth of the system will be limited by the excitation of parasitic modes other than the TM011. This limitation will cause a rather limited span of measurable dielectric layer thicknesses.
Therefore, there is a need for methods and handy devices that are capable of non destructive measurement of a thin film thickness over substrate material which has a medium electrical conductivity and may be anisotropic, such as for use in the production and maintenance of air plane parts made of carbon fibre reinforced plastic with and without metallic mesh for lightning protection. Furthermore, generally the dielectric constant of the film is not known and sometimes also the substrate material and its conductivity are not exactly known to the inspector. Thus there is a need for methods and devices that do not heavily depend on these possibly unknown parameters.